1. Field of the Invention
The present invention relates to nonaqueous electrolyte secondary batteries having positive-electrode active materials and negative-electrode active materials, which intercalate/deintercalate (or are doped/undoped with) lithium, and nonaqueous electrolyte solutions.
2. Description of the Related Art
Nickel-cadmium batteries and lead batteries have been used as secondary batteries for electronic devices. Trends toward higher performance and miniaturization of electronic devices due to advanced electronic technology require secondary batteries having higher energy densities. Since nickel-cadmium batteries and lead batteries have low discharge voltages, increases in the energy densities are limited.
Nonaqueous electrolyte secondary batteries using carbonaceous materials capable of intercalating/deintercalating lithium in negative electrodes and lithium compound oxides in positive electrodes have been vigorously developed instead of the nickel-cadmium batteries and lead batteries, since the nonaqueous electrolyte secondary batteries, called lithium ion batteries, have high discharge voltages and reduced self-discharge, and have prolonged cycle lives.
In these nonaqueous electrolyte secondary batteries, carbonaceous materials such as graphite are used as negative-electrode active materials, LixMO2 wherein M is at least one transition metal and 0.05 less than x less than 1.10 is used as positive-electrode active materials, and LiPF6 and LiBF4 are used as electrolytes. As organic solvents for dissolving the electrolytes, propylene carbonate, ethylene carbonate, xcex3-butyrolactone, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, ethyl acetate, methyl propionate, 1,2 -dimethoxyethane, and 2-methyltetrahydrofuran are used.
The nonaqueous electrolyte secondary batteries are suitable as power sources for portable electronic devices. In recent years, compact battery packs including batteries and protective circuits have been frequently used with requirements for reduced sizes and weight. In the batteries in the battery packs, higher capacities at large-current discharging modes are required. In order to fulfill such a requirement, improvements in negative electrodes are essential in addition to improvements in positive electrodes and nonaqueous electrolyte solutions.
Current nonaqueous electrolyte secondary batteries, however, are still unsatisfactory as regards improvements in capacities during large-current discharging modes by improvements in negative electrodes.
Accordingly, it is an object of the present invention to provide a nonaqueous electrolyte secondary battery having satisfactory capacity characteristics during a large-current discharging mode.
According to an aspect of the present invention, a nonaqueous electrolyte secondary battery comprises a positive electrode comprising a positive-electrode active material, a negative electrode comprising a negative-electrode active material, the positive-electrode active material and the negative-electrode active material capable of intercalating/deintercalating lithium, and a nonaqueous electrolyte solution, wherein the negative electrode further comprises carbon fibers and carbon flakes.
The carbon fibers and carbon flakes can be disposed in the interstices between the negative-electrode active material particles in this configuration. Moreover, the carbon fibers improves retention of the nonaqueous electrolyte solution and the carbon flakes disposed between the active material particles improves conductivity (reduces internal resistance). These synergistic effects improve the capacity characteristics of the nonaqueous electrolyte secondary battery. In this nonaqueous electrolyte secondary battery, lithium is smoothly doped or undoped at large-current charge or discharge conditions, resulting in high capacity at the high-current load.
In this nonaqueous electrolyte secondary battery, the content of the carbon fibers in the negative electrode is in a range of preferably 0.02 percent by weight to 5 percent by weight and more preferably 0.5 percent by weight to 4 percent by weight, and the content of the carbon flakes in the negative electrode is in a range of preferably 0.1 percent by weight to 30 percent by weight, more preferably 1 percent by weight to 20 percent by weight, most preferably 1 percent by weight to 10 percent by weight.
Moreover, the ratio by weight of the carbon fibers to the carbon flakes in the negative electrode is in a range of preferably 0.2 to 100 and more preferably 0.4 to 20.
In a preferred embodiment of the present invention, the nonaqueous electrolyte secondary battery comprises an electrode composite in which a positive electrode including a lithium compound oxide as the positive-electrode active material and a negative electrode including a carbonaceous material as the negative-electrode active material are wound with a separator disposed therebetween (called a jelly roll type).